Concrete Slab Calculator

What this calculator does

This concrete slab calculator focuses on flat, on-grade pours: patios, driveways, sidewalks, shed floors, and equipment pads. Enter length, width, and thickness in your preferred units, set a waste percentage, and the calculator returns volume in cubic yards, cubic feet, and cubic meters, plus 40 lb, 60 lb, and 80 lb bag counts. Add a price per cubic yard for a cost estimate. For footings, columns, walls, or stairs, use the main Concrete Calculator.

Formula

Volume: V_ft³ = length_ft × width_ft × thickness_ft

Multi-pad totals: V_total = V_one × quantity

Order with waste: V_order = V_total × (1 + waste)

Conversions: V_yd³ = V_ft³ ÷ 27 · V_m³ = V_ft³ × 0.0283168

Bag count: bags = ⌈V_ft³ ÷ bag_yield × (1 + waste)⌉ · 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³.

Cost: cost = V_order_yd³ × price_per_yd³

Variable definitions

• length / width — Slab footprint in your chosen unit — converted to feet internally.
• thickness — Slab depth, typically entered in inches (4 in for patios/sidewalks, 5–6 in for driveways and pads).
• quantity — Number of identical slabs to pour (e.g., 4 deck-post pads).
• waste — Extra ordered for over-pour and form variation, as a decimal (0.05 = 5%, 0.10 = 10%).
• bag yield — Cured volume per bag: 0.30 ft³ for 40 lb, 0.45 ft³ for 60 lb, 0.60 ft³ for 80 lb (Quikrete / Sakrete equivalents).

Step-by-step calculation

1. Pick a preset (patio, sidewalk, driveway…) or enter custom dimensions.
2. Confirm units for length, width, and thickness — the calculator converts to feet internally.
3. Enter quantity if you're pouring multiple identical pads.
4. Set the waste factor — 5% for clean rectangles on a flat sub-base, 10% for irregular forms.
5. Optionally enter price per cubic yard for an itemized cost.
6. Order ready-mix by the cubic-yard figure (rounded up to the supplier's increment) or bagged mix by the rounded-up count.

Worked example

12 ft × 10 ft patio, 4 in thick, 10% waste:

• Thickness in feet: 4 ÷ 12 = 0.333 ft
• Cubic feet: 12 × 10 × 0.333 = 40 ft³
• Cubic yards: 40 ÷ 27 = 1.48 yd³
• Order volume (with 10% waste): 1.48 × 1.10 = 1.63 yd³
• 80 lb bags: ⌈40 ÷ 0.60 × 1.10⌉ = 74 bags
• 60 lb bags: 98 bags · 40 lb bags: 147 bags
• At $160 / yd³: 1.63 × $160 = $260.80

How to use this calculator

1. Tap a slab preset for typical thickness, or set thickness manually.
2. Enter length and width in feet, inches, yards, meters, or centimeters.
3. Adjust the waste factor and quantity.
4. Add a price per cubic yard if you want a cost estimate.
5. Use the bag counts to plan a trip to the home center, or the cubic-yard figure to schedule a ready-mix delivery.

Common mistakes

• Mixing units: thickness is usually in inches while length and width are in feet — the calculator converts but the wrong unit will silently 12× the result.
• Skipping waste: coming up short on a slab pour means a cold joint or a costly short-load fee.
• Rounding bags down: always round up to whole bags.
• Forgetting the sub-base: volume calculations assume the form is filled to the top with concrete. A poorly graded sub-base eats extra mix.

Frequently asked questions